In general, fibers derived from natural materials such as cellulosic fibers and animal fibers (hereinafter referred to also as natural fibers) are more excellent in hygroscopicity and water absorbability than synthetic fibers. However, when washed in water, the natural fibers tend to be swollen and thereby be hardened, embrittled, or whitened. Furthermore, the natural fibers are disadvantageously inferior in wrinkle resistance and strength to the synthetic fibers.
Therefore, there is a demand for modifying the natural fiber without deteriorating its inherent hygroscopicity and water absorbability, thereby producing a modified fiber having washing durability, strength, and the like equal to those of the synthetic fiber. For example, a method for providing sheep wool in a natural fiber with water-repellent/oil-repellent properties is proposed in Japanese Laid-Open Patent Publication No. 08-134780. Specifically, a wool fiber is subjected to an oxidation treatment, and a polysiloxane resin such as dimethylpolysiloxane and a fluorine compound such as a polytetrafluoroethylene resin are adsorbed in this order to the wool fiber, to form a water-repellent/oil-repellent coating. However, in this case, the adhesion is insufficient between the wool fiber and the coating. Thus, the coating is often peeled off in a washing process or the like, whereby the water-repellent/oil-repellent properties are often deteriorated.
In view of the above problem, for example, Japanese Laid-Open Patent Publication No. 2008-202174 discloses an animal hair fiber containing sheep wool, a water-repellent/oil-repellent coating containing a fluorine-containing acrylate resin or the like, and an intermediate coating layer formed therebetween, which contains a polyamide-epichlorohydrin or the like capable of forming a covalent bond with the animal hair fiber. In this case, because the covalent bond is formed between the intermediate coating layer and a functional group in the animal hair fiber, the adhesion between the water-repellent/oil-repellent coating and the animal hair fiber is improved by the intermediate coating layer, whereby the water-repellent/oil-repellent properties last longer.